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Six-membered heterocycles containing one phosphorus and one nitrogen atom, known as azaphosphinines, have existed in the shadows of their single heteroatom-containing analogues for almost 150 years. Despite this, recent chemistry has seen a rapid increase in publications concerning this uncommon scaffold. Azaphosphinines exist in one of six isomers—there are three possible orientations of the pnictogen atoms and in each of these, the phosphorus is in one of two valences (PIII vs. PV). This review aims to outline and inform on the synthesis and applications of all six isomers. PV–oxo azaphosphinines are of particular interest to this review as many of the discussed heterocycles either form as the pentavalent species directly or oxidize to this over time. In very recent years the published applications of azaphosphinines have blossomed into subjects spanning several fields of chemistry such as asymmetric catalysis, supramolecular association, cellular imaging, and medicinal chemistry. 

Several phosphaquinolinone derivatives have been synthesized and characterized to explore their usefulness in the realm of cell imaging. Solution-state photophysical properties in both aqueous and organic solutions were collected for these derivatives. Additionally, CCK-8 cell viability assays and fluorescent imaging in HeLa cells incubated with the new heterocyclic derivatives show evidence of favorable cell permeability, cell viability, and moderate intracellular localization when appended with the well-known morpholine targeting motif. 

Abstract Lawesson's reagent is one of the most common thionating reagents that has found use for several decades on a variety of carbonyl systems. Herein, we report the transformation of the 2‐λ⁵‐phosphaquinolin‐2‐one structure to its respective 2‐λ⁵‐phosphaquinolin‐2‐thione derivative. Solution‐state characterization of both P−OPh and P−Ph thio analogues, as well as solid‐state structures for the latter system, are reported.